This invention relates to a seal system for a turbomachine and in particular, to a turbomachine wherein the seal fluid employed to maintain the working medium within the working region of the machine is completely miscible within the working medium.
As is well-known to those skilled in the art, the utilization of seals on turbomachines, such as compressors and expanders or turbines, is extremely important to maintain the working fluid of the machine within the machine's working region, and simultaneously to prevent the flow of contaminants into the machine from the ambient. Very often the seals are one of the most critical elements of the machine.
The recent fossil fuel shortage has created the need for alternative sources of energy. Geothermally heated fluids are considered to be one of the most likely candidates for producing a viable source of energy. Many of the recognized leaders in the field of power recovery from geothermally heated fluids have promoted the use of what is known in the art as a "binary system." A binary system involves the vaporization of a secondary heat transfer fluid, generally an organic fluid such as isobutane, by passing the secondary fluid in heat transfer relation with the geothermally heated fluid. The vapor thus formed is delivered to a turbomachine, whereat the vapor is expanded to generate power. The expanded vapor is thereafter condensed for repeat of the cycle.
As the cost of the secondary fluid is relatively high, it is essential that the seals employed on the expander be extremely reliable to prevent loss of the vaporous hydrocarbon fluid.
In many seals used in other applications, the sealing medium may either be steam or air. However, a seal employed in an expander using an organic or inorganic fluid as the working medium cannot employ either steam or air. Condensed steam can contaminate the working medium. Air cannot be used as a sealing with a medium expander using an organic fluid as the working medium, since a mixture of air and organic fluid is extremely explosive. An example of a combination steam and air seal is disclosed in U.S. Pat. No. 3,302,951. As indicated above, the system disclosed in this patent cannot at all be employed with an expander utilizing an organic or inorganic fluid as the working medium.